Work has continued in Dr. Bertino's laboratory centering around folate antagonist mechanism of action and B12-folate interrelationships. Two new approaches to folate antagonist chemotherapy developed in these laboratories in recent years will soon be brought to clinical trials: Carboxypeptidase G1, an enzyme that produces folate depletion in vivo and NSC 139105 (Triazinate), a new folate antagonist that crosses the blood brain barrier. This latter compound was found to be effectively concentrated by Walker 256 cells in vivo and in vitro, but not by L1210 cells; this finding correlated with the marked effectiveness of this drug against the Walker 256 tumor, and its ineffectiveness against mice bearing the L1210 lymphoma. Human tumors are being screened in a similar manner to detect sensitivity to Triazinate. A program has been initiated which has as its objective the elucidation of the synthesis of polyglutamate folate forms in liver and brain, and evidence has been obtained to indicate that reduction of folic acid is not a prerequisite for polyglutamate formation. Work has continued on the purification and comparative biochemistry of dihydrofolate reductase from human normal tissues (liver, erythrocytes) and from neoplastic tissues (leukemia). Mechanisms of resistance to folate antagonists both in experimental tumors and in man, and the regulation of folate antagonists both in experimental tumors and in man, and the regulation of folate enzymes continue to also be important areas of research for this laboratory.